Frankly, this question only comes up because Turbotax thinks you are stupid. The IRS only needs to know the amount of sales tax you paid. If you are using the standard allowance method, the IRS has a table that assigns an amount of sales tax you are allowed to claim based on your income and location. It's probably a fair estimate. You can only add to the standard allowance, sales tax you paid for the purchase of a motor vehicle aircraft, a home, or a major renovation to the home. (There is no "major purchase" rule, it is only those 4 specific things.)
Instead of Turbotax asking for the sales tax from your bill of sale, Turbotax thinks you can't look that up, so it asks for the purchase price of the car so it can calculate the rate. Sometimes, this can actually lead to a wrong answer. If you buy a $40,000 car with a $10,000 trade in, your purchase price that you pay sales tax on is usually $30,000, not $40,000, so if you answer $40,000, you will get a false deduction.
In your case, if you buy a car for $40,000 with some combination of cash down payment and a loan, then your purchase price is $40,000.
But again, you can't deduct title and license fees, only the sales tax.
It would be better if Turbotax simply asked for the amount of sales tax you actually paid.
